The present invention relates generally to gas turbine engines, and, more specifically, to balancing of rotors therein.
Gas turbine engines include various rotors in the typical form of bladed disks. Each rotor disk is specifically configured with a radially outer rim from which extends a row of blades. An axially thinner web extends radially inwardly from the rim and terminates in an axially thicker hub having a central bore therein.
The disk is circumferentially continuous and has substantial hoop strength for withstanding the centrifugal loads developed by the blades as they rotate during operation about a longitudinal or axial centerline axis of the disk. The disk shape maximizes the strength thereof while minimizing undesirable weight for effectively supporting the blades over a substantial service life.
The rotor disks have various forms for supporting relatively large fan rotor blades and multiple rows of compressor blades decreasing in size for compressing air during operation. The air is mixed with fuel and ignited for generating hot combustion gases which flow downstream through various rows of turbine blades increasing in size on corresponding rotor disks therefor.
In one common configuration, the rotor disks include either axial-entry dovetail slots through the rim thereof, or a common circumferential-entry dovetail slot which correspondingly receive complementary blade dovetails for retention to the disks. In another common configuration, the blades may be integrally formed with the rim of the disk in a unitary or one-piece construction typically referred to as a blisk which is an acronym for the unitary bl(aded-d)isk.
The advantage of the dovetail construction is the ability to individually manufacture the blades and disk, and permit simple repair thereof by disassembly of the blades from the disk. However, the dovetail construction requires a correspondingly larger disk for withstanding the various pressure and centrifugal loads experienced during operation.
A particular advantage of the blisk construction is that the integral disk may be smaller since no dovetails are used, and the blades are integrally formed around the disk rim. However, this increases repair difficulty since the blades are not readily individually removable from the disk. Minor repairs of the blade may be made in the blisk, but major repair thereof requires removal by cutting of corresponding portions of damaged blades or their complete removal, with the substitution thereof being made by welding or other metallurgical bonding process for achieving the original strength of the blisk.
An additional difficulty in the manufacture of the blisk is balancing thereof. All rotor components in a gas turbine engine must be suitably statically and dynamically balanced for minimizing rotary imbalance loads during operation for reducing vibration. The dovetail disk construction permits the rotor to be initially balanced during manufacture, with the individual blades being separately manufactured and matched in position on the disk for minimizing the resulting imbalance of the assembly thereof. The assembled disk may then be conventionally balanced using various forms of balance corrections.
In contrast, the typical manufacturing tolerances in the manufacture of the individual blades of the blisk result in corresponding variation in the respective mass or weight thereof. This in turn creates an overall imbalance of the blisk which must be suitably corrected.
Conventional balancing machines measure imbalance of the blisk at a suitable speed in terms of an imbalance force vector having magnitude in mass and radius, and at a measured circumferential angular position around the circumference of the blisk relative to any suitable reference point. The measured imbalance may be corrected by either removing blisk material at the angular position of the imbalance vector or by adding additional material diametrically oppositely therefrom, such as at 180xc2x0 for example.
Material may be removed from identified blades, or in the platform region between blades. Material may also be removed from flanges on corresponding extension shafts of the blisk which are used for carrying torque load thereto from the low pressure turbine of the engine which powers the blisks.
However, identifying suitable locations for removing sufficient mass of material increases the difficulty of the balancing process since material removal may decrease the ultimate strength of the blisk which must be maintained at a suitably high level for ensuring a suitable useful life.
The addition of a suitable amount of material for balancing the blisk presents its own difficulties. It is typically not practical to add material to the blades or disk itself without adversely affecting the strength thereof. Instead, balance correction in the form of discrete balance weights may be added at the mating flanges of the blisk.
Accordingly, it is desired to provide a blisk having an improved balance feature.
A blisk includes a disk having a rim from which extends a row of blades. The rim includes axially opposite overhangs one of which includes an arcuate balance land disposed eccentrically to the centerline axis of the disk for balancing the blisk.